“I realized this natural event was a rare opportunity to record rigorous experiments that could have broad implications on how we care for our forests. That’s big.”

- John Wenzel, Director of Powdermill Nature Reserve

Friends of the Forest

To clear or not to clear? Powdermill sets in motion a study of tornado-damaged forests.

By Julie Hannon

When a tornado struck Ligonier in June of 2012, about 90 acres of trees were toppled on and near Powdermill Nature Reserve, the environmental research center of Carnegie Museum of Natural History. John Wenzel, director of the reserve, took a day or two by foot to survey the damage. Because it was extensive and followed a specific pattern, Wenzel got to thinking during his walk through the debris: This could be good.

Pitt graduate student Jake Slyder pauses at the edge of a harvested section of forest prior to setting up deer fences.

Good, that is, for learning how wind-damaged forests rejuvenate. As it turns out, while there’s a long history of studying how forests respond to major wind disturbances like tornados, virtually no solid research exists about how clearing fallen timber afterwards impacts forest recovery in the eastern United States, says Wenzel, an entomologist who primarily studies social insects.

“I realized this natural event was a rare opportunity to record rigorous experiments that could have broad implications on how we care for our forests,” says Wenzel. “That’s big.”

His first call was to Walter Carson, a biologist at the University of Pittsburgh who has been studying the forests of the eastern part of the country for 30 years. Carson and a team of collaborators recently wrapped up 14 years of research in the Allegheny National Forest demonstrating what the Bambi-loving public may find shocking: that white-tailed deer have a much greater negative impact on the health of a forest than deep shade or even fire.

“Overbrowsing by deer often determines the pattern of forest rejuvenation in the east, and overbrowsing can cause biodiversity collapses,” says Carson. “They are that bad.”

Carson jumped at Wenzel’s proposal of a partnership, knowing Powdermill is perfectly positioned for such a groundbreaking study for two big reasons. Unlike most other private landowners, the museum is willing to leave money on the ground, so to speak, forgoing revenue from logging in the name of science. And in 2008, the reserve conducted a landscape survey of its forests—which includes 45 species of trees—so researchers know the measurements of the forest prior to the wind damage, an incredibly helpful and highly unusual bonus.

“Timber can be very valuable, so loggers and the Forestry Service see it as an important resource going to waste if you leave it on the ground,” says Carson. “The counterargument from ecologists is that downed logs contain vital nutrients for the soil; that salvaging trees and creating a flat landscape disturbs habitats for birds and other animals. Both schools of thought depend on your [college] degree. What is needed is good science to resolve debate.”

After all, the country needs wood products, he says, and millions of acres of land is salvaged, or cleared of fallen timber, across the country each year. If science can help inform best practices, it would be a significant win-win.

One important question researchers want to answer is whether a salvaged or unsalvaged forest will lead to greater rejuvenation, and what species will grow back? When many trees are downed due to a tornado, the increased light at ground level can lead to the rapid growth of plants that would struggle under the shade of a forest’s canopy. But is it possible that timber left on the ground will inhibit such growth, physically obstructing its development? Or, could the difficulty of walking through an often dangerous, maze-like mess of fallen trees inhibit deer foraging, giving some plants more of a chance to survive where they would have had little or no chance in a cleared forest?

“That’s what makes this study so unique and the most complete of its kind—that we’ll study the impact of deer browsing along with the impact of salvaging and the impact of the understory, all simultaneously,” Carson says.

The pair, in partnership with researchers from Ohio State, Penn State, University of Wisconsin-Milwaukee, University of Vermont, Virginia Tech, and Susquehanna University, expect to have initial findings within two or three years. But they plan to keep the experiment running for at least a decade in hopes that it will become a landmark location of forest recovery that’s reexamined for 50 years or more.

James Whitacre, the reserve’s GIS manager, gets a bird’s eye view to document the experiments.

To perform their experiments, the team will begin by using two 10-acre sections that are in close proximity to each other and of similar makeup—both important controls. Half of each will be salvaged. Equal parts of both the salvaged and unsalvaged areas will then be fitted with deer exclusion fences. And finally, in half of each deer-excluded area, the low-lying, deer-tolerant forest cover known as the understory will be removed, giving all plants across the test areas equal footing to grow. Then the team will replicate the experiments.

What do they expect to find? “Our prediction is that the best way to provide a diverse, highquality forest decades after a blowdown is to salvage the timber, build fences to eliminate deer browsing, and remove the surviving understory when it sprouts again,” Wenzel says.

But whatever the findings, he notes, they’ll inform how we responsibility care for our forests. Putting into perspective just how much this matters: There were more trees blown down by Hurricane Katrina in just the state of Mississippi than the entire United States cut down for any reason in 1995, the year of that devastating storm.

“We’ll tease out the major factors,” says Carson. “You have to be willing to do the big experiments and do them correctly. And it takes a unique place like Powdermill to make this thing run.”